Users of the PoC service may initiate a 1-1 (one-to-one), 1-M (one-to-more) or 1-M-1 (one-to-more-to-one) session through a PoC mobile phone. In the 1-M or 1-M-1 mode, multiple users participate in the session. When initiating a session, an initiator may adopt an Ad hoc (temporary) mode, or a Pre-arranged (predefined) mode. In the Ad hoc mode, the initiator temporarily selects a participant from a contact person list to initiate a group session; and in the Pre-arranged mode, group information is defined before a user initiates a session, and the initiator initiates a group session only with a group identification.
A user may be invited in two scenes as follows: firstly, a participant is designated when a session is established and in this case, either the Ad hoc mode or the Pre-arranged mode may be adopted; secondly, a session participant invites a user to join in the way of adding a participant during the session.
The existing PoC session includes a sender end signal procedure and a receiver end signal procedure. In those two procedures, an invitation information is described with a SIP invite request or a SIP re-invite request. A session request message that is sent to a PoC server by a PoC session initiating end contains session participant information and initiator information. A session request message that is sent to an invited user by the PoC server contains initiator information, but no session participant information.
According to Open Mobile Alliance (OMA) PoC standards, there exit two kinds of servers in the PoC session, i.e. a participating function server and a controlling function server. The controlling function server is in charge of centralized session control and media distribution. The participating function server cooperates with the controlling function server to control a session and store a user's session configuration. The session configuration includes the user's answer modes—automatic answer mode or manual answer mode, coming call screening and personal instant message screening etc. A calling user may override the use's answer mode if necessary. When the use's answer mode is a specified answer mode—automatic answer mode or manual answer mode, the calling user may put relevant description information into an invitation, so as to reverse the use's answer mode. In other words, the use's manual answer mode may become the automatic answer mode; or the automatic answer mode may become the manual answer mode. The premise for the override operation is that the called user has granted the calling user right to override the use's answer mode. A corresponding authentication will be performed by the participating function server.
In the PoC session, the invited user may decide whether to participate in the session according to the session group member information. Because a exiting session request message sent to the invited user doesn't contain the session group member information, the invited user may not know the group information before participating in an Ad hoc session. When the invited user finds that there is a member whom the invited user is unwilling to communicate with after participating in the session, and then quits, unnecessary cost may be caused.